1. Field
Apparatuses and methods consistent with exemplary embodiments relate to a capacitive deionization apparatus and a method for manufacturing the same, which capacitive deionization apparatus is enhanced in the removal efficiency for ionic substances and the fluid throughput, hence applicable to water with high salt concentration such as sea water, etc., and easy to manufacture.
2. Description of the Related Art
Capacitive deionization (CDI) is a technology directed to removing water of ionic substances by using the adsorption and desorption of ions in the electric double layer (EDL) formed at a charged electrode interface.
FIG. 1 is a diagram showing the principle of the CDI technique, illustrating the process of ion adsorption and desorption on the surface of charged electrodes. Referring to FIG. 1, upon applying a voltage at a potential difference range that does not incur electrolysis of water, the electrodes are charged with a defined quantity of electric charges. As a stream of brine water containing ions flows through the charged electrodes, the counter-ions having the opposite charge sign to the one in the charged electrodes move to each electrode by the electrostatic force and become adsorbed from the water onto the surface of the electrodes, and the water passing through the electrodes turns to desalinated water that is free from ions.
In this regard, the quantity of ions adsorbed onto the electrodes depends on the capacitance of the electrodes in use. Thus, the electrodes as used in the CDI are generally porous carbon electrodes with a large specific surface area.
When the electrodes are saturated with the ions beyond their adsorption capacity, the electrodes cannot adsorb ions anymore and the ions of the influent stream become stuck in the effluent stream. In order to release the ions adsorbed by the electrodes, a short-circuit potential or an opposite electric potential to the adsorption one is applied to the electrodes. This causes the electrodes to lose electric charges or get counter charges, and the adsorbed ions are rapidly released from the electrodes, leading to a regeneration of the electrodes.
In this manner, the CDI technique is very easy to operate, for the technique involves adsorption and desorption of ions triggered merely by changing the electric potential of the electrodes. Further, the CDI causes no emission of environmental pollutants during the deionization, so it is thus known as an eco-friendly deionization process. As an example of the improved CDI, the membrane capacitive deionization (MCDI) device includes an ion exchange membrane formed on the surface of the electrodes in order to increase the selectivity for ions to adsorb.
However, the CDI or MCDI electrodes of the related art, which are made up of a stationary active material (e.g., activated carbon, carbon fiber, carbon aerogel, etc.), have a limitation in the ion adsorption performance. In order to secure a large adsorption capacity, it is necessary to expand the electrodes to a large area or to stack multiple electrodes. However, the above-described expansion of the electrodes can cause a great increase in the costs for the manufacture and operation of the related equipment in need.
Accordingly, many attempts have been made to use a novel material with high ion adsorption performance as an electrode material. Such methods, however, not only require a complicated process for the manufacture of electrodes but also have a limitation in maximizing the surface area of the novel material due to formability, density, etc.
In the CDI or MCDI of the related art, it is common to design the flow channel so narrow as much as about 100 μm in order to increase the deionization efficiency. Such a narrow flow channel is susceptible to heavy fouling, reducing the water throughput and deteriorating the productivity. Further, the narrow flow channel makes it difficult to make a serial capacitive deionization (CDI) module with large area, leading to a limitation in raising the productivity.
Particularly, when increasing the water throughput for the sake of commercialization in the CDI or MCDI of the related art, it is necessary to assemble a plurality of unit cells required to realize a defined capacity. This can deteriorate the productivity and also increase the volume of the apparatus, causing a limitation in the mobility of the apparatus. In addition, the capacitive deionization (CDI) apparatus of the related art possibly causes the channeling effect on the surface of the electrodes in the stack depending on the influent and effluent positions of the fluid and thus deteriorates the ion removal efficiency.